The effect of the ratio of reactant molecular diameter to catalyst pore diameter (lambda) on the restrictive diffusion under hydrodesulfurization (HDS) reactions of heavy residue oils over CoMo/Al2O3 catalysts was investigated. A series of Al2O3 with various pore structures were used as supports of CoMo sulfide. The HDS reaction was carried out in a semi-batch Carberry type reactor at 648 K and 10.3 MPa. Two empirical correlations for restrictive diffusion under HDS reaction conditions were developed depending on the value of lambda. The results showed that the restrictive diffusion effect under HDS reaction conditions is severe for the lower values of lambda. However, this effect seems not prominent for higher values of lambda. The order of the magnitude of the effective diffusivity was in the range of 10(-6)-10(-7) cm(2)/s for the sulfur-containing compounds of residue oil in CoMo/Al2O3 catalysts at 648 K.